[Mimedefang] Asymmetric postmaster notify count on two cross-MX'd servers

[Kenneth Porter]

I have two servers on different ISP's that act as backup MX for each other. 
As near as I can tell they're identically configured except for domain 
name. Each has MD 2.42, ClamAV 0.70, and SpamAssassin 2.63. MD is set to 
reject messages with a SA score of 10 or more.

One gets a ton (30 a day, maybe) of "postmaster notify" messages from 
rejected bounces (typically "no such user") while the other gets almost 
none. Most of the spam is for the host that gets none. What could be 
causing the asymmetry? Is it just because so much more ratware that uses 
open relays attacks backups that the primary for the spam victims rarely 
sees spam from open relays?